The invention relates to a fuel injection system having a pressure step-up unit.
For the sake of better comprehension of the specification and claims, some terms will first be defined: The fuel injection system of the invention can be embodied as either a stroke-controlled or a pressure-controlled fuel injection system. Within the scope of the invention, the term stroke-controlled fuel injection system will be understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and a control chamber. A pressure reduction inside the control chamber causes a stroke of the valve member. Alternatively, the excursion of the valve member can be effected by a final control element. In a pressure-controlled fuel injection system according to the invention, as a result of the fuel pressure prevailing in the nozzle chamber of an injector, the valve member is moved counter to the action of a closing force spring, so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which the fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or kept on hand inside the fuel injection system. The term fuel metering means the furnishing of a defined fuel quantity for injection. The term leakage is understood to mean a quantity of fuel that occurs in operation of the fuel injection system, for instance a reference leakage which is unused for injection and is returned to the fuel injection system. The pressure level of this leakage can have a static pressure, whereupon the fuel is then depressurized to the pressure level of the fuel injection system.
A stroke-controlled injection has been disclosed for instance by German Patent Disclosure DE 196 19 523 A1. The attainable injection pressure here is limited by the pressure storage chamber (rail) and the high-pressure pump to approximately 1600 to 1800 bar.
To increase the injection pressure, a pressure step-up unit is possible, of the kind disclosed for instance by U.S. Pat. No. 5,143,291 or U.S. Pat. No. 5,522,545. The disadvantage of these pressure-stepped-up systems resides in a lack of flexibility of the injection and poor quantity tolerance in the metering of small fuel quantities.
In a fuel injection system described in Japanese Patent Disclosure JP 08277762 A, two pressure storage chambers with different pressures are provided in order to enhance the flexibility of injection and increase the metering precision of the preinjection. These two pressure storage chambers require major production effort and high production cost, and the maximum injection pressure is still limited by the fuel pump and the pressure storage chamber.
A pressure step-up unit disposed in the injector is known from European Patent Disclosure EP 0 691 471 A1. A bypass line for a pressure injection and a pressure chamber of the pressure step-up unit are in line with one another, so that the bypass line is open only as long as a displaceable piston of the pressure step-up unit is not moved and is completely retracted.
The fuel injection system of the invention enhances the flexibility and increases the maximum injection pressure. Each injector of a common rail system is assigned a hydraulic pressure step-up unit, which enables both increasing the maximum injection pressure to higher pressure, such as greater than 1800 bar, and furnishing a second, higher injection pressure. The bypass line leads at the end of the pressure chamber of the pressure step-up unit into the lead line to the nozzle chamber or into the lead line of the pressure step-up unit to the nozzle chamber. An injection of fuel at lesser pressure can be effected regardless of the position of the pressure means in the pressure step-up unit. By means of the pressure step-up unit, the pressure storage chamber and injector are subjected to a lesser static pressure (rail pressure) and thus have a longer service life. The high-pressure pump also suffers less stress. The possibility exists of a meterable preinjection with low tolerances by means of low (non-stepped-up) injection pressure. By switching over between the injection pressures, a flexible postinjection or a plurality of postinjections at high or low injection pressure are feasible.